This invention relates to a cable structure of a submarine cable system employing low-loss optical fibers as transmission media in the field of optical communication and a method for the manufacture of such a submarine cable.
Because of its advantages of such as a low loss, a wide transmission band and a light weight, an optical fiber is regarded as a promising transmission media by which a coaxial submarine cable heretofore employed will be replaced.
A submarine cable is laid under the deep sea of up to 10,000 m and exposed to a water pressure as high as about 1,000 atm. at maximum. When an optical fiber cable is received such a high pressure, the optical fiber slightly bends due to nonuniformity in the thickness of a material coated thereon, resulting in markedly degraded transmission characteristic. Further, the optical fiber is made of silica glass or optical glass of small loss and hence is brittle and may be broken when greatly bent. Since the optical fiber is thus made of glass, when it is immersed in sea water for a long period of time, its mechanical property and transmission characteristic are degraded. To avoid this, it has already been proposed to house the optical fiber for the submarine cable in a high pressure resisting pipe (Japanese Pat. Disc. No. 99032/76). Since the high-pressure resisting pipe increases its thickness with an increase in its inner radius, it is necessary in the manufacture of the pressure resisting pipe to minimize its diameter. In a case of inserting one optical fiber into the pipe, its diameter may be reduced, but in a case of inserting a plurality of optical fibers in the pipe, reduction of the pipe diameter is difficult if the pipe is adapted to be used both as a pressure resisting pipe and a power feeding conductor or the like. In the optical-fiber submarine cable, it is necessary from the economical point of view to increase the number of optical fibers utilizing their small diameter as well as to transmit a large quantity of information utilizing the wide transmission band property of the optical fibers themselves. It is also possible to employ such a method of increasing the number of fibers by protecting each optical fiber with a pipe but, in this case, a space loss by division occurs in the manufacture of pipes. In view of the above, it is desirable to house a plurality of optical fibers in one pressure resisting pipe, but in the prior art, since an increase in the inner diameter of the pipe causes an increase in the pipe thickness, it is difficult to manufacture a thick pipe while inserting optical fibers at the center thereof.